In the field of robotic object manipulation as, for example, in a parcel handling system of a post office distribution center, there is the need to be constantly aware of the distance between a manipulating arm and the object to be moved. A target ranging apparatus for use in such a system would ideally be compact, lightweight, inexpensive, yet reliable and accurate to within a few millimeters, over a span of distances from several centimeters to a few meters.
In general, there are two techniques in widespread use for target ranging. The first technique involves assailing the target with radiant energy, as with a pulsed or chirped laser beam, or with acoustic energy, detecting the signal reflected from the target, and comparing the relative phases of the transmitted and received optical or acoustic signals. Such a system is disclosed in U.S. Pat. No. 3,954,340, "METHOD OF AND APPARATUS FOR TARGET TRACKING," issued May 4, 1976, to A. H. P. Blomquist et al. This form of target ranging tends to be too complex and expensive for the type of system contemplated.
The second technique involves a triangulation measuring scheme, in which light from a source is reflected by the target to a position on a detector, and the distance to the target is reckoned by the length of the baseline between the source and the reflected target image at the detector. This technique is widely used and is the range finding scheme disclosed in U.S. Pat. No. 4,340,302, "ENDOSCOPE WITH SENSOR," issued July 20, 1982, to T. Oku, and in U.S. Pat. No. 4,477,108, "RANGE FINDER," issued Oct. 16, 1984, to K. Hosoe.
Both of these techniques require a priori knowledge relating to the source of illumination. In the first instance, the phase of the transmitted signal must be known for comparison with the phase of the received signal. In the second instance, the position of the transmitted illuminating signal must be known since the computed distance to the target is a direct function of the difference between the positions of the illuminating source and the received image.
The present invention employs neither of these techniques; rather, it makes use of the fact that a location on a target object, when illuminated by a narrow beam of light, will act as a point source of light radiation, and that this point source, when imaged by a lens onto a plane detector surface located away from the focal point of the source, forms a blurred circle whose diameter can be directly related to the distance between the lens and the point source. Thus, the range of the object reflecting the light radiation can be determined solely by measuring the diameter of the blurred circle, which is independent of the positions of the illuminating source and the position of the image on the detector.